The nonaffine transient network theory is used to study the time development of the shear and normal stresses under start-up shear flows in networks formed by self-assembled telechelic, hydrophobically modified water-soluble polymers. The initial slope, strain hardening, and overshoot of the shear stress are studied in detail in relation to the nonlinear tension-elongation curve of the elastically active chains in the network. The condition for the occurrence of strain hardening (upward deviation of the stress from the reference curve defined by the linear moduli) is found to be gamma > gammac(A), where gamma is the shear rate, gamma(c) is its critical value for strain hardening, and A is the amplitude of the nonlinear term in the tension of a chain. The critical shear rate gamma(c) is calculated as a function of A. It is approximately 6.3 (in the time unit of the reciprocal thermal dissociation rate) for a nonlinear chain with A = 10. The overshoot time t(max) when the stress reaches a maximum and the total deformation gamma(max) = gamma(t max) accumulated before the peak time are obtained in terms of the molecular parameters of the polymer chain. The maximum deformation gamma(max) turns out to depend weakly upon the shear rate gamma. The first and second normal stress differences are also studied on the basis of the exact numerical integration of the theoretical model by paying special attention to their overshoot, undershoot, and sign change as a function of the shear rate. These theoretical results are compared with recent rheological experiments of the solutions of telechelic hydrophobically modified poly(ethylene oxide)s carrying short branched alkyl chains (2-decyl-tetradecyl) at both ends.

The collapse of a poly(N-isopropylacrylamide) (PNIPAM) chain upon heating and the phase diagrams of aqueous PNIPAM solutions with a very flat lower critical solution temperature (LCST) phase separation line are theoretically studied on the basis of cooperative dehydration (simultaneous dissociation of bound water molecules in a group of correlated sequence), and compared with the experimental observation of temperature-induced coil-globule transition by light scattering methods. The transition becomes sharper with the cooperativity parameter σ of hydration. The reentrant coil-globule-coil transition and cononsolvency in a mixed solvent of water and methanol are also studied from the viewpoint of competitive hydrogen bonds between polymer-water and polymer-methanol. The downward shift of the cloud-point curves (LCST cononsolvency) with the mol fraction of methanol due to the competition is calculated and compared with the experimental data. Aqueous solutions of hydrophobically modified PNIPAM carrying short alkyl chains at both chain ends (telechelic PNIPAM) are theoretically and experimentally studied. The LCST of these solutions is found to shift downward along the sol-gel transition curve as a result of end-chain association (association-induced phase separation), and separate from the coil-globule transition line. Associated structures in the solution, such as flower micelles, mesoglobules, and higher fractal assembly, are studied by ultra small-angle neutron scattering with theoretical modeling of the scattering function. Dynamic-mechanical modulus, nonlinear stationary viscosity, and stress build-up in start-up shear flows of the associated networks are studied on the basis of the affine and non-affine transient network theory. The molecular conditions for thickening, strain hardening, and stress overshoot are found in terms of the nonlinear amplitude A of the chain tension and the tension-dissociation coupling constant g.

Succinoglycan samples ranging in weight-average molecular weight from 1.0 x 10(5) to 8.7 x 10(6) (in 0.1 M aqueous NaCl at 25 degrees C), prepared by ultrasonication of a native sample (Rheozan), followed by fractionation, were investigated by static light scattering, sedimentation equilibrium, and viscometry in 0.1 M aqueous NaCl at 25 degrees C where the polysaccharide assumes a certain ordered (helical) conformation. The measured radii of gyration and intrinsic viscosities showed the polysaccharide to behave like a semirigid chain in the aqueous salt. Their analysis based on the unperturbed wormlike chain yielded about 1500 nm-1 and 50 nm for the linear mass density and the persistence length, respectively. The former value was almost twice that expected for the single succinoglycan molecule, and thus it was concluded that the predominant molecular species of succinoglycan present in the aqueous salt is a double helix or an aggregate composed of paired single helices.

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Small-angle X-ray scattering (SAXS) from dual-surfactant aqueous solutions made from sodium lauryl ether sulfate and coconut fatty acid amido propyl betaine was systematically measured as a function of the net sodium cation concentration, [Na + ]*, and the surfactant concentration, C D . The SAXS intensity [I(q)] was normalized to C D and the resultant I(q)/C D was extrapolated to C D = 0 to give a form factor P(q) for each [Na + ]* [where q = 4 sin( /2)/ is the magnitude of the scattering vector, is the wavelength and 2 is the scattering angle]. The low-q behaviour of P(q) was consistent with long rigid cylinders. The middle-and high-q profiles fitted well with a core-shell cylinder model for all [Na + ]*. The core and total radii (R c and R s ) did not depend on [Na + ]* at all: R c = 1.2 AE 0.05 and R s = 3.1 AE 0.05 nm for [Na + ]* = 0.42-1.5 mol l À1 , indicating that the salt concentration changes did not induce any structural changes and reassembling of the surfactants comprising the micelles. This fact is in contrast to the rheological behaviour where the relaxation mode strongly depends on [Na + ]*. The structure factor [S(q)] was obtained by dividing I(q)/C D by P(q) for each C D and the mean distance (d m ) between the micelles was obtained from the first maximum of S(q) versus q plots. The d m value decreased with increasing C D and [Na + ]*, which is in good agreement with the theoretical prediction and experimental results for charged wormlike micelle solutions.